This invention relates to a mooring/support system for marine structures. In this specification marine structures include submerged structures or structures which float or are supported above or near the surface from the sea bed, and include both mono-hull and semi-submersible type ships, barges, oil or gas drilling and production platforms and vessels, towers, fish nets, pens, or devices which may support communications and radar equipment, navigational aids, or any equipment needed to be positioned at sea. Reference herein to a ship or vessel is accordingly to be taken as meaning any marine structure.
The oil and gas industry in particular have a need for positioning operational ships in a specific location relative to the sea bed so that operations such as drilling for oil or gas, recovering and processing the oil or gas, mining and exploring the ocean or its sub-surface can be achieved. The ships or platforms would hold all the equipment necessary for completing such operations. In addition they would hold the equipment needed for communications and radar operations navigational aids, etc.
When such shipping is used in the production of oil or gas for example, it is essential to control the location of the ship during all environmental conditions within strict tolerances which are required by the characteristics of the connection for the flow of oil and gas between the ship and the sea bed. The limitation may be because of mechanical or operational restrictions.
It is known to anchor a mono-hull or semi-submersible ship using single or multiple conventional chain link or wire cable anchor lines extending downwardly from the ship in one or several directions, the anchor lines being connected to anchors in the sea bed. A riser system, which may be flexible, semi-rigid or rigid, is used for connecting the operational equipment onboard ship to sea bed equipment such as the wellhead of a subsea oil well for example. Turret mooring is provided on a mono-hull type ship to receive the mooring chains and risers while allowing the ship to rotate about the anchorage to take into account tides, winds, currents, etc, without twisting the anchor lines and risers. Uni-directional shaped ships, such as semi-submersibles, can be moored in a fixed orientation because the environmental forces act the same in all directions on the vessel; therefore these types of ships do not need a turret.
Another known marine structure used in the oil and gas industries is the tension leg platform (TLP) which is a semi-submersible type vessel that has post-tensioned anchoring tendons extending substantially vertically from the bottom of the hull to anchor points in the sea bed. This type of platform relies on considerable lateral movement to develop the horizontal restraining force it needs to stay near location. The magnitude of this movement depends on sea depth and weather conditions.
With guyed tower type structures which extend downwardly and engage with the sea bed, lateral support for the top of the tower is provided by flexible guy lines which are fixed to the upper portion of the tower and which are inclined and extend downwardly to the sea bed anchors. There can be several levels of guys.
Self supporting structures are founded on the sea bed, supporting operational platforms above the sea thereon with a structure which diverges outwards and engages with the sea bed. The structure is made of steel or concrete or a combination of both to provide the necessary strength and rigidity. Normally these platforms support the drilling or process equipment above the sea and extend down to the sea floor. The risers, and other connections to the wells are usually contained in the support structure.
In yet another known system in which the ship is dynamically positioned, the ship remains on station during all weather conditions utilizing a position monitoring device which determines the ship's exact location. The monitoring device is connected to a computer controlled propulsion system which repositions the ship when the control system senses movement away from the designated point. The system must have the capabilities to maintain the ship in position during all weather conditions. Consequently, not only are costs high for operation, maintenance, and propulsive power but also the initial investment is large. Furthermore, positive location maintenance is not attained because there is no physical restraining device connecting the ship to the sea bed. Neither can positive system reliability be absolutely assured if a critical component fails.
When a ship is positioned or supported by conventional anchorings or guys, the lateral stabilization or horizontal restoring force is provided by the horizontal component of the tension force in the anchor line. However, because of the inclination of the anchor lines an additional vertical downward force is imposed upon the ship. To counter the effects of these adverse vertical forces additional buoyancy may need to be provided in the case of floating structures while with supported towers additional strength and stiffness may need to be provided to accommodate the extra vertical load.
Additionally, forces are produced from external sources such as waves, wind, and current that also act upon the anchor lines imparting additional loadings on them. Although these loadings are usually normal to the direction of the main axis of the elements of the anchor lines they introduce amplified axial tension forces into those inclined lines which have horizontal and vertical components which introduce secondary forces on the ship or structure. Any detrimental effect caused by these secondary forces must be accounted for and this leads to additional cost and complexity.
A conventionally anchored mono-hull or semi-submersible ship requires specific equipment and facilities for the handling and storing of the anchor lines. When the water depth increases, a greater amount of anchor line is required and not only must the space necessary to store such anchor lines be provided in the ship, but also the equipment must be made larger and sturdy enough to be able to handle the movement of such large lengths of anchor lines, thus increasing the complexity of the facility. Increased water depth for both conventionally anchored ships and guyed towers tends to increase the flexibility of the entire system and this too has detrimental effects or limitations on the practicability of means of positioning the vessel within the required tolerance. In some cases performance can be improved by the addition of servo-controlled tensioning equipment, but this too increases complexity and reduces reliability.
In the case of tension leg platforms (TLP) it is necessary to introduce exceptionally high downward pretension forces which complicates anchoring by having to cater for the higher uplift on the sea bed foundations. The TLP's horizontal restoring forces are only induced by the inclination of the anchoring tendons which requires movement of the floating vessel or platform to resist external forces such as waves, winds, currents etc. This contradicts the purpose of the anchorage which is to reduce or eliminate movement.
Self supporting structures founded on the sea bed become increasingly complex and expensive as water depth becomes greater. This type of structure is not technically feasible or cost effective for conducting exploration and production activities in deeper ocean in areas of limited life and economic return.
It is an objective of the present invention to provide a mooring/support system which is simple in construction, but which is capable of accurately maintaining a ship on location with maximum reliability and safety. It is also an object of this invention, by providing a separate mooring or support, to minimize the cost and complexity of the moored or supported object, thus reducing total costs.
It is a further object of the present invention to provide a mooring/support system which can be pre-installed. That is to say, one which can be positioned, and can remain, on site independently of the marine structure as well as one that can be utilized by other vessels during other phases of a project.